Nuclear engine reactor rocket cores

ABSTRACT

A nuclear rocket engine reactor core equipped with a series of coolant channels having hydraulic diameters that decrease in programmed proportion with increasing radial position from the core center to thereby reduce the required propellant coolant flow to a minimum during the aftercooling period occurring after reactor shutdown.

United States Patent [191 Moon [ Feb. 26, 1974 NUCLEAR ENGINE REACTORROCKET CORES [75] Inventor: Calvin W. Moon, Greenhills, Ohio [73]Assignee: The United States of America as represented by the Secretaryof the Air force, Washington, DC.

22 Filed: July 23,1968

[21] App1.No.: 747,784

[52] US. Cl. 60/203, 176/59 [51] Int. Cl G21d [58] Field of Search60/203; l76/56, 59

[56] References Cited UNITED STATES PATENTS 3,108,054 l0/l963 Blackman,Jr. 60/203 4/1965 Vamn et al. 176/59 11/1966 Plebuch 60/203 PrimaryExaminer-Samuel Feinberg [5 7] ABSTRACT A nuclear rocket engine reactorcore equipped with a series of coolant channels having hydraulicdiameters that decrease in programmed proportion with increasing radialposition from the core center to thereby reduce the required propellantcoolant flow to a minimum during the aftercooling period occurring afterreactor shutdown.

4 Claims, 10 Drawing Figures PATENTEUHMB m I I 3.793.832

' sum 3 0F 3 I INVENTOR. fll VIA w. Mao/v 1 NUCLEAR ENGINE REACTORROCKET CORES BACKGROUND OF THE INVENTION This invention relatesgenerally to nuclear-powered rocket engines and, in particular, toimproved means for achieving more uniform temperatures in the nuclearreactor core during the after cooling period.

The ever increasing development in the nuclear propulsion fieldinvolving the use of nuclear reactors as heat sources for providing thepropulsion means involves, among others, the problem of reducing theheat occurringparticularly in the exit nozzle area of the engine after aparticular power operation has ceased, or in other words, after thereactor has been shut down; of course, this problem could be eliminatedby designing the nuclear-powered engine for a one time operation only.However, such a solution would be too costly and if one is to designsuch an engine with a restart capability, the structural integrityparticularly in the nozzle itself and the surrounding engine structuremust be preserved through some prefereably simplified cooling means.

The present invention generally solves this problem of aftercooling witha minimum of complexity being added to the'overall engine system by theaddition of extra hydrogen propellant to the main propellant supply tankfor the engine. A unique system of coolant passages is utilized in animproved combination arrangement, as will be hereinafter discussed inthe following summary and detailed description.

SUMMARY OF THE INVENTION A principal object of the present invention,therefore, resides in the development of a new and improved nuclearrocket engine-reactor assembly having a uniquely-arranged and moresimplified combination preferably consisting of a single reactorpressure shell incorporating a series of individual side reflectormodule elements in novel arrangement with an improved reactor core madeup of a plurality of variable porosity and variable hydraulic diameterfuel elements to thereby effect an improved nuclear rocket performanceduring the after cooling operational mode.

A further object of the invention is in the utilization of a novelnuclear reactor-powered rocket engine having, in a preferred embodimentthereof, a main reactor pressure shell element formed as an integralforward extension of the engine nozzle and incorporating a plurality ofcoolant tubes and further being in unique combination with a new andimproved reactor core having variations in both coolant passage spacingsand diameters arranged in a predetermined manner to effect an increasedaverage temperature of the coolant propellant at the exit from the coreduring the aftercooling period.

Other objects and advantages of the invention will become readilyapparent hereinafter in connection with the following disclosure, inwhich:

BRIEF DESCRIPTION OF THE DRAWING somewhat schematic mountingarrangements that may be made between the pressure shell element of theoverall assembly of FIG. 1, and each of a series of coolant tubesutilized therewith;

FIG. 2 is a front end view, partly broken away and schematic in form, ofthe assembled engine of FIG. 1, illustrating further details of aspecific arrangement between a series of side reflector module elementsthat may be supported on the outside circumference of the reactorpressure shell, and additionally, illustrating a relationship therewithof one type of control drum actuator and flexible tube elements that maybe respectively used to control the neutron emission from the reactorcore, and the transmission of coolant between certain of the reactorassembly units.

FIG. 3 is a cross sectional rear end view, partly broken away, generallyillustrating additional details of the nuclear reactor powered rocketengine of FIG. 1 and, in particular, more clearly showing the specificand unique relationships between the major reactor assembly componentsutilized with the overall assembly view of FIG. 1;

FIG. 4 represents a detailed upright view of a forward tube sheetelement that may be used'with the assembled view of FIG. 1 forsupporting the reactor core-fuel elements of the present invention intheir correct position;

FIG. 5 is a second view of the forward tube sheet element of FIG. 4,partly broken away and cross sectional in form, and taken about on line5-5 thereof, and generally illustrating more clearly the overallconfiguration thereof, and in particular, showing the formation ofcounterbored holes therein utilized to provide the support for the oneend of the reactor-core fuel elements;

FIG. 6 is a longitudinal view, partly broken away and schematic in form,showing details of a representative example of one type of the severalfuel elements forming the active portion of the inventive reactor coreportion of FIG. 1; and

FIG. 7 is a side perspective and schematic view of the fuel element ofFIG. 6, showing one cross-sectional configuration thereof and furtherillustrating a plurality of coolant passages incorporated therein.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing and, inparticular, to FIGS. 1 and 2 thereof, one type of nuclear-powered,rocket engine reactor-shield assembly utilizable with the presentinvention is indicated generally at 10 as including an exit nozzlestructure 11, a main reactor pressure shell 12 that surrounds the activereactor-core portion, a series of side reflector modules indicatedgenerally at 13 (Note FIG. 2), each of which includes a control drumactuator indicated generally at 14, a forward or front nuclear radiationor heat shield element 15, a front tube sheet element 16, and an activereactor-core portion indicated generally at 17, which core portion 17incorporates a number of novel fuel elements indicated generally andschematically at 18 and which actually comprise the active portion ofthe said reactor core. The aforesaid modules 13 are disposed incircumferential relation to, and completely surround the outercircumference of the main reactor pressure shell 12 (Note FIG. 2).

The above noted reactor pressure shell 12 is actually formed byextending the nozzle structure 11 in a forward direction throughout theentire length of the reactor. In this manner, fabrication of the presentarrangement is greatly simplified by the elimination of the usualproblem required in joining the exit nozzle structure with the pressureshell. The outer part of the forward flange portion of the main reactorpressure shell 12 may be supported by the forward or front shieldelement 15, whereas the inner part of the said flange pottion joins thefront tube sheet element 16 which may be flexibly supported to providedifferential radial expansions. The aforementioned supporting and/orconnecting elements are not shown in detail, since they form no part ofthe present invention.

The previously-mentioned main reactor pressure shell 12 is formed with aplurality of open-ended, thinwalled coolant tubes which are brazed tothe inner wall surface thereof. These pressure shell coolant tubes,several of which are indicated at the reference numeral 19, are arrangedin closely packed and surrounding relation to the reactor core 17 andassist in providing a series of continuous coolant passages extendingforwardly from the region of the exit nozzle structure 11 to a plenum inthe heat shield element 15, as is illustrated at the reference numeral20. In this connection, a portion of the nozzle structure 11 may beregeneratively cooled in a well known manner, as for example, by meansof incorporating within the outer walls thereof a plurality of coolingtubes, passages or channels, one of which is indicated at the referencenumeral 21 in FIG. 1. In this regard, an extra supply of hydrogenpropellent may be stored in the main engine pressure supply tank (notshown) and introduced into the aforesaid nozzle coolant tubes 21 by wayof the nozzle inlet manifold at 22. Main reactor pressure shell 12 isdesigned to resist internal loads due to a pressure differential that isformed across it and also because of the formation of certain thermalstresses. To facilitate resistance to the aforesaid internal loads,three mounting arrangements may be provided between the shell itself andthe previously noted coolant tubes 19. In the first such arrangementshown in FIG. 1a, coolant tube 19a is depicted as being affixed to thesurface of a single pressure shell at 12a. This mounting meanssubstantially represents the embodiment used in the present invention.In FIG. lb, however, a split shell configuration is used which consistsof the inner and outer shells indicated at 12b and 120. With thisarrangement, an interior coolant passage 19b is formed therebetween. Inthe third form of FIG. 1c, a combination of both the split shellarrangement of FIG. lb, as depicted at 12d and 12e, and the single shellform of FIG. 1a is utilized to thereby respectively form both aninteriorily disposed coolant passage at 19c and a coolant tube at 19d.In each of the aforesaid mounting means, the resulting internal loadsformed therewithin are depicted by means of a series of arrows, whichare self explanatory.

The front or forward shield element 15, previously described inconnection with FIG. 1, is actually formed into a plurality ofindividual, circular heavy metal plate elements which may be weldedtogether along adjacent peripheries thereof to thereby eliminate anyrequirement for an additional external pressure vessel. A plurality ofsmall coolant passages, indicated generally at 23, are formed within theaforesaid shield element to thereby permit the passage of the hydrogencoolant therethrough prior to its entry into the reactor. These coolantpassages 23 are staggered to reduce radiation streaming. Moreover, theinner portion of the said shield element acts as structural backbone forthe present reactor assembly. Thus, both the present reactor andpressure shell elements are supported from the shield element 15, andthe engine thrust created is transmitted through the shield element tothe thrust structure. In addition, the inner portion of the said shieldelement also supports the outer, individual shield plate segments, eachof which in turn supports a separate side reflector module 13, as isillustrated in FIG. 1. The local thermal stresses in the shield element15 may be readily controlled through the proper selection of thediameter and spacing of the coolant passages 23.

As noted hereinbefore, the preferred embodiment includes a plurality ofindividual side reflector modules 13 (Note FIGS. 2 and 3). There is aseparate side reflector module 13 exteriorly mounted to the outercircumference of each of the previously indicated individual circularplate segments welded together and collectively comprising the frontshield element 15. With particular reference to FIGS. 1 and 3, it isseen that each separate reflector module 13 actually constitutes anintegral package containing the necessary supporting structureconsisting of an outer reflector module pressure shell or pipe 24, whichencloses the nuclear reflector material indicated generally at 25 inFIG. 3 and, in addition, is supported from a particular individual platesegment of the heat shield element 15, previously described. Thisreflector material 25 is actually further enclosed within a cylindricalcontrol drum element 26, which control drum element 26 contains bothneutronrefiecting material that is generally indicated at the aforesaidreference numeral 25, and neutron-absorber material indicated at thereference numeral 27, which in one embodiment may consist of a series ofneutron absorbing rods formed in relatively enlarged holes incorporatedwithin each control drum element 26.

The aforesaid reflector structure-pressure shell or pipe 24 is designedto resist the differential pressure between the hydrogen propellant andthe vacuum of space. Moreover, each of said shells or pipes 24incorporate a plurality of small coolant tubes 28, brazed to the insidecircumference and along the periphery thereof. These coolant tubes 28are actually within the space provided between the pressure shell orpipe 24 and the control drum element 26, and they are provided to insurenearly uniform skin temperatures circumferentially around the outermoststructure of each module 13.

The previously mentioned control drum element 26 effects reactor controlby being rotatable within the aforementioned outer pressure shell orpipe 24 to thereby position the absorber material 27 incorporatedtherewithin relative to the active core portion 17 of the presentreactor. The neutron-reflecting material 25 may consist of the metalberyllium, since the latter material permits a simple and lightweightdesign and may also serve as the control drum structure itself. Aplurality of axial and relatively elongated holes or passages may beprovided in the aforesaid beryllium material as illustrated at 29 (FIG.3) for the purpose of permitting the flow of hydrogen coolanttherethrough. The previously described neutron absorber material or rods27 are located in a series of relatively large holes or passages,illustrated by reference numerals 27, and which are positioned alongapproximately of the total periphery of each control drum element 26, asis clearly indicated in the aforesaid FIG. 3. In this manner, accurate,simple and flexible control of the active core portion of the presentreactor may be easily effected.

Bearings for the control drum element 26 are located at each endthereof, as is generally indicated at the reference numerals 26a and2612 (FIG. 1), and the axial thrust produced is transmitted throughthese bearings. If it is found nuclearly desirable, for better reactorcore control purposes, the void between adjacentlypositioned sidereflector modules 13, and the main reactor pressure shell 12, may bereduced by the use of filler blocks shown at 35 (FIG. 3). These fillerblocks 35 each consist of a thin wall tube containing neutron reflectingmaterial and are supported in position by adjacent modules 13. Axiallyoriented passages may also be incorporated in the tiller blocks 35, asshown at 35a, again for the purposes of permitting the flow of coolanttherethrough, which coolant may be supplied from the module plenums inany appropriate manner.

The previously referred to control drum actuator 14 (FIG. 1) may be ofan already developed design used in an existing program and, althoughits specific structure is not important to the present invention and istherefore not shown in detail, it generally consists of a pistonactuator whose linear motion is converted to a 180 rotary motion bymeans of a rack and pinion method. Normally, this control actuator actsagainst the force of a scram spring indicated at 30. However, saidactuator is also designed to assist in the scram action to provide addedprotection in the unlikely event of a spring failure. As seen in theaforesaid FIG. 1, each of the control actuators 14 is actually, in thepresent embodiment, an integral part of a respective side reflectormodule 13 and is naturally operative to control the rotative position ofthe control drum element 26 and, of course, the neutron-reflecting andabsorbing material incorporated therewithin relative to the reactor coreitself as previously noted. Also, as clearly illustrated in theaforesaid FIG. 1, the supporting structure indicated generally at 31 and32 respectively supports the control drum actuator 14 and the controldrum actuating shaft 33 to thereby form an integral part of acorresponding side reflector module 13 and, in the case of the structure31, to actually provide the support of the module 13 and control drumactuator 14 to the corresponding individual plate segment of the shieldelement 15. The area along which these two elements are cojoined isindicated generally by the reference numeral at 34. Moreover, each ofthe aforesaid control drum actuator-supporting structures 31incorporates a series of coolant passages or tubes 36 which maycommunicate with the already described passages 28 formed in each module13.

With specific reference to FIGS. 4 and 5, one form of the reactor fronttube sheet which may be used with the present invention is showngenerally at 16 as consisting of a circular plate element, perforatedwith a plurality of circular holes 36, which holes are counterbored(Note FIG. 5) to relatively short depths in order to accommodate orreceive the forward or front ends of the corresponding fuel elements 18comprising the active portion of the reactor core. The outer peripheryof the tube sheet 16 may be machined to reduce its total weight and toprevent large temperature differences to exist across the radius of thetube sheet. Said tube sheet 16 may be further attached to the innerportion of the radiation shield element by means of a structuralcylindrical element which may be adapted to provide both support for thetube sheet itself and to permit differential radial expansion relativeto the main pressure shell 12. Also the connection therebetween may bewelded to assure sealing between the various coolant flow passages. Theforegoing elements are not illustrated in detail since they form no partof the present invention. The aforesaid tube sheet 16 may be cooled bythe direct passage therethrough of hydrogen coolant during the passageof the latter material from the coolant passages 23 formed in the shieldelement 15 into the inlet end of the active portion of the reactor core.

The aforementioned active portion of the reactor core 17 of the presentinvention consists of the previously noted plurality of fuel elements18, which fuel elements are fastened within the previously described,counterbored circular holes 36 formed in the reactor front tube sheet16. Each of the said fuel elements 18, one of which is shown in detailin the view of FIG. 6, and in schematic form in FIG. 7, consists of anactive core section at 18a, a nose piece element at 18b and a tail pieceelement at 18c. It is this nose piece 18b which is fastened within theholes 36 in the reactor tube sheet 16 as generally shown by the means at36a in FIG. 1. On the other hand, the rear or aft ends of each of saidplurality of fuel elements are interconnected to each other to maintainthe proper clearance between each respective element thereof.

The active core or fueled section of each fuel element 18 may contain orconsist of an appropriate matrix material, such as W-UO which may beclad with a material such as a referactory metal alloy. The phenomenonknown as reactor radial power flattening is accomplished by a reductionin the fuel concentration therein toward the centerline of the activecore. The latter effect is accomplished by changing the percentage of U0or other material in the fuel matrix. Since the fuel element shown isbuilt up axially with a number of fueled segments, axial power tailoringthrough fuel variations may be also readily accomplished.

As stated hereinabove, the aforesaid nose piece 18b, at the forward endof the fuel element 18, connects the particular element against'or tothe forward tube sheet 16. For this purpose, a threaded fastener orother similar element, such as is illustrated generally at 37 in FIG. 1,may be used to seal the face of each nose piece against the forward tubesheet 16. Furthermore, an integral form of a seal ring (not shown) maybe utilized to position or locate the nose piece of each fuel element inthe appropriate counterbored hole 36 provided therefor in the said tubesheet. Furthermore, the aforesaid nose piece may be machined and joinedto its particular fuel element 18 through means of diffusion bonding.The tail piece 18c may also be configured to thereby transition from aperforated plate form to a circular ring form connected with theadjacently positioned tail piece element of each of the plurality offuel elements 18 to thus provide radial restraint therebetween. Again,each tail piece may be joined to the fueled section 18a by means ofdiffusion bonding.

The foregoing series of fuel elements, 18, which form a unique featureof the present invention, are arranged in a closely packed assembly thatis hexagonal in cross section. However, other cross-sectionalconfigurations are usable with the present invention. Further, each fuelelement 18 incorporates or is pierced with a series of coolant holes orpassages, as indicated generally at 38 in the schematic fuel elementform of FIG. 7. These channels may be triangularly or otherwise arrangedin a matrix of refractory metal and oxide fuel. The improved feature ofthe aforesaid coolant passages 38 is that, in accordance with theteachings of the present invention, the amount of coolant required tocounteract the excessive increase in afterheat resulting after reactorshutdown may be minimized in a novel manner by programming the size anddistribution of the aforesaid coolant passages 38 in accordance with theactual heat generated. Thus, the present invention performs this uniqueprogramming function by equipping the previously described fuel elements18 with a series of coolant passages as at 38 in FIG. 7 which are variedin both size and distribution to match or substantially match the localheat generated in the fuel elements 18 in accordance with, or inmeasured proportion to the radial distance from the center of the core.To this end, and to correspond with previously computed locat heatgeneration in the reactor core at various radial distances, the presentinvention provides the above noted matching capability between the localheat generated and the capability to efficiently remove the heat sogenerated to respectable and acceptable temperature ranges, by teachingthe utilizing of relatively larger coolant holes 38 near the center ofthe reactor core and then progressively decreasing the size of suchcoolant passages as the outer surface of the core is approached. In thismanner, the amount of additional hydrogen coolant required to be usedfor afterheat purposes may be significantly reduced and more efficientspace missions accomplished. Of course, the foregoing objective ofminimizing the required coolant may be alternatively accomplished eitherby using uniform hole spacings, in which event, fuel loading will haveto be increased in regions of larger hole sizes to increase the localpower density, or if the hole spacings are varied with the radialdistance in addition to the variation in hole size, the necessarycoolant minimization can be achieved in a radially power flattened core.Of course, as a practical matter, rather than vary either the hole sizeor spacing within a single fuel element, small temperature excesses dueto small power variations may be more acceptable than the increasedtooling and fabrication costs. In any event, the present inventionteaches that, by utilizing larger than average coolant channels, as at'38, at the center of the reactor core, followed by decreasing sizecoolant channels towards the core periphery and/or additionally varyingthe number or spacing of the channels outwardly from the reactor corecenter, not only will the required coolant be materially reduced butalso a net effect of achieving higher and therefore more efficientaverage core temperatures will be the result throughout the reactor coreduring aftercooling. The latter is accomplished in the present inventionspecifically by using the same number of coolant channels 38 for eachfuel element 18 and decreasing the sizes of the holes in each element asthe distance increases outwardly from the core center except near thecore boundary when the power increases. The latter configuration iseffected for each fuel element 18, except for the peripheral fuelelements. In the latter elements, the hole or channel sizes 38 arevaried as in the other fuel elements but the number of holes or channels38 incorporated therein are increased as a means of reducing the largestholes to sizes commensurate with the typical sizes for the whole reactorcore. In this regard, although in the design tested only two holespacings were utilized, it is obvious that other specific arrangementsapplicable to different reactor core designs could be used withoutdeparting from the true spirit or scope of the present invention.Accordingly, because of the resulting increased core boundarytemperature, the heat transferred to the counter-flowing coolant in thesurrounding core shell and therefore the temperatures of the coolantentering the core itself are increased. Thus, with the same localtemperature ratios as limited by laminar flow characteristicsexperienced during aftercooling, the level of the radial maximum exittemperature can be increased with further decreases in the requiredcoolant flow rate. The latter effect naturally increases the efficiencyof the nuclear powered rocket engine, since the radial temperaturedistribution across the core of the reactor for rocket applicationshould be as uniform as possible in order to achieve as high an exit gastemperature as possible. The present invention uniquely achieves thisobjective by radially varying the hydraulic diameters of the coolantpassages, such as at 38, across the reactor core as previously noted tothereby achieve a uniform radial temperature distribution.

With the foregoing arrangement, therefore, improved aftercooling iseffected by the combined effect of the propellant coolant, such ashydrogen, initially flowing from the nozzle-coolant tubes at 21forwardly through the pressure shell-coolant tubes 19, from whence itenters each of said modules 13, by way of a first, relatively shortflexible tube at 39, into the coolant tubes 28 provided in the spaceformed between the module-outer pressure shell 24 and the outerperipheral wall of the control drum element 26. This coolant flow entersthe aforesaid module coolant tubes 28 at a position ahead of the shieldelement 15 to thus flow in a rearward or aft direction therethrough andimmediately thereafter reverse its direction of flow to then passthrough both coolant passages 29 provided in the control drum element 26and the control shaft 33, which is hollow in form to thereby providestill another coolant passage. From this point, the coolant flowproceeds or exits through a second, relatively elongated, flexible tube40 to thereafter continue in its coolant flow through the coolantpassages 23 formed in the heat shield element 15 by way of the lattersplenum at 20. The aforesaid hydrogen coolant passes through the saidshield passages 23 and enters the reactor core to pass therethrough byway of the previously described, programmed coolant passages 38 uniquelyarranged so as to provide the necessary aftercooling with a minimumexpenditure of propellant.

While a preferred embodiment of the present invention has been shown anddescribed for purposes of exemplification, it is apparent that manymodifications and changes may be made without departing from the truespirit and/or scope of the invention, as defined hereinafter in theaccompanying claims.

I claim:

1. In a nuclear rocket engine, reactor-shield assembly; an exit nozzleportion; a main reactor having a pressure shell; an active reactor coreportion contained within said main reactor pressure shell; a heat shieldelement attached to, and supporting said main reactor pressure shell;combined neutron-reflecting and neutron-absorbing material means forcontrolling said active reactor core; and combined propellant-and- 2. Ina nuclear rocket engine, reactor-shield assembly as in claim 1, whereinsaid plurality of coolant channels are formed and equally spaced in aseries of reactor core-fueled elements, each having different hydraulicdiameters of predetermined variable sizes measured outwardly towards thecore periphery to thereby provide for the flow of coolant therethroughsubstantially matching and dissipating the local heatgenerated atvarious radial distances during the reactor shutdownaftercooling periodand also to counteract the reduced peripheral temperatures caused by theincrease in relative heat loss to the surrounding core structure.

3. In a muclear rocket engine, reactor-shield'assembly as in claim 2,wherein the coolant channels formed in said fueled-elements vary inspacing from one element to the other in all of said elements except forthe peripheral fueled elements.

4. In a nuclear rocket engine, reactor-shield assembly as in claim 3,wherein said active reactor core portion incorporates peripheral fueledelements having variablesized coolant channels greater in number thanthat of the remainder of said fueled elements to thereby more nearlyequalize the larger-sized holes formed therewithin and typicalthroughout the whole reactor core.

1. In a nuclear rocket engine, reactor-shield assembly; an exit nozzleportion; a main reactor having a pressure shell; an active reactor coreportion contained within said main reactor pressure shell; a heat shieldelement attached to, and supporting said main reactor pressure shell;combined neutron-reflecting and neutron-absorbing material means forcontrolling said active reactor core; and combinedpropellant-and-coolant continuous flow passage means incorporated withinsaid reactor-shield assembly providing a continuous flow of propellantand coolant in said assembly between said exit nozzle portion and thereactor inlet, said active reactor core portion incorporating aplurality of coolant channels of variable hydraulic diameters decreasingin the direction of the core periphery in accordance with a programmedsequence to thereby match the decreasing heat generated in the reactorcore at increasing radial distances from the core center.
 2. In anuclear rocket engine, reactor-shield assembly as in claim 1, whereinsaid plurality of coolant channels are formed and equally spaced in aseries of reactor core-fueled elements, each having different hydraulicdiameters of predetermined variable sizes measured outwardly towards thecore periphery to thereby provide for the flow of coolant therethroughsubstantially matching and dissipating the local heat generated atvarious radial distances during the reactor shutdown-aftercooling periodand also to counteract the reduced peripheral temperatures caused by theincrease in relative heat loss to the surrounding core structure.
 3. Ina muclear rocket engine, reactor-shield assembly as in claim 2, whereinthe coolant channels formed in said fueled-elements vary in spacing fromone element to the other in all of said elements except for theperipheral fueled elements.
 4. In a nuclear rocket engine,reactor-shield assembly as in claim 3, wherein said active reactor coreportion incorporates peripheral fueled elements having variable-sizedcoolant channels greater in number than that of the remainder of saidfueled elements to thereby more nearly equalize the larger-sized holesformed therewithin and typical throughout the whole reactor core.